Nathan, Comparing a 590 to an 870 is like comparing apples and oranges. One is an auto loader and the other a pump.
While I think a Mossberg gives you good value for the money, I went with the 870 because of the alloy receiver of a 500. Is that the one that can get tweaked by over tigtening the screws when attaching a side saddle to the receiver? It probably doesn't happen very often, but recently there was a thread posted by a guy that somehow managed to jam his new 500 action up solidly by forcing the slide back. He is waiting to have Mossberg figure out just what went wrong, but my guess it wouldn't have happened or would not have happened as easly if it was a steel receiver.